A large, thin, and light waterproof case such as a battery case is equipped with a safety apparatus configured to prevent the waterproof case from bursting as a result of an increase in internal pressure thereof. Examples of a general safety apparatus include a pressure relief valve and a rupture disc. However, these safety apparatuses are not actuated unless a certain level of pressure is applied to the apparatus because a portion of the apparatus which is subjected to pressure is small in size with respect to the case.
For airtight cases with low pressure resistance, the pressure relief valve has a significantly weak spring force, and the rupture disc has a significantly small film thickness. Thus, due to disturbances other than the pressure in use (for example, vibration, acceleration caused by impact or the like, and incoming fragments such as dust) or aging (corrosion or degradation of a material caused by ultraviolet rays) the pressure relief valve or the rupture disc may be actuated at a pressure equal to or lower than a pressure which needs to be withstood by the pressure relief valve or the rupture disc. Consequently, the airtightness may be lost.
On the other hand, for waterproof cases with low pressure resistance, a waterproof breathing filter (porous PTFE film) is often used to prevent leakage and damage to the case resulting from a difference in pressure between the inside and outside of the case. However, the waterproof breathing filter is subjected to a heavy pressure loss associated with gas transmission and thus has a limited gas transmission rate. Thus, the waterproof breathing filter fails to deal with a rapid change in the volume of an internal gas. When the limit is exceeded, the pressure in the case is higher than the withstanding pressure of the case, which may thus burst. As is the case with airtight cases with low pressure resistance, many filters are small in size, and many filters themselves have a bursting pressure higher that the withstanding pressure of the case.
The small waterproof breathing filter may be blocked from any cause, for example, clogging with dust, freezing, or contamination with oil. In this case, the small waterproof breathing filter may fail to breath, similarly making the pressure in the case higher than the withstanding pressure of the case.